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Serum phosphate is an important determinant of corrected serum calcium in end‐stage kidney disease
Author(s) -
FERRARI PAOLO,
SINGER RICHARD,
AGARWAL AANCHAL,
HURN ANNE,
TOWNSEND MARY A,
CHUBB PAUL
Publication year - 2009
Publication title -
nephrology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.752
H-Index - 61
eISSN - 1440-1797
pISSN - 1320-5358
DOI - 10.1111/j.1440-1797.2009.01121.x
Subject(s) - calcium , albumin , phosphate , bicarbonate , serum albumin , medicine , endocrinology , kidney disease , chemistry , biochemistry
SUMMARY Background: Approximately 12% of bound blood calcium is linked to various anions including phosphate. In patients with end‐stage kidney disease (ESKD), serum phosphate is highly variable. We propose that establishing a formula to calculate albumin‐ and phosphate‐corrected total calcium would be more appropriate to estimate free calcium in ESKD patients. Methods: In 82 haemodialysis patients, serum ionized calcium (Ca ion ) and pH were measured by blood gas analyser with ion‐selective electrodes at the point‐of‐care, while bicarbonate, phosphate, albumin, magnesium and total calcium (Ca tot ) were measured at the central laboratory. Linear regression analysis of measured variables was used to best fit adjusted calcium versus Ca ion . Results: The most parsimonious multiple linear regression model ( r 2 = 0.81) of variables associated with Ca ion included Ca tot (coeff 0.820, P < 0.0001), albumin (coeff −0.016, P < 0.0001) and phosphate (coeff −0.063, P < 0.002). Modelling of available variables yielded the following equation to adjust calcium for albumin and phosphate: Ca albPh = Ca tot + (0.015 × (40 − [albumin]) + 0.07 × (1.5 − [phosphate])). At an ambient albumin of 40 g/L, Ca albPh would be 0.07 mmol/L lower than Ca tot for every mmol/L of phosphate. In vitro data using three different albumin levels and increasing phosphate concentrations demonstrated this relationship, with the slope of the phosphate effect being stronger at lower albumin concentrations. Conclusion: Because guidelines recommendations indicate that corrected serum calcium should be maintained within the normal range in ESKD patients, inclusion of phosphate to correct Ca tot in these patients may have clinical implications on the choice of phosphate binders and the prescription of vitamin D or calcimimetic agents.